1. Field of the Invention
The present invention relates to color processing utilizing a profile. More particularly, the invention relates to a technique for executing calibration processing suited to a profile.
2. Description of the Related Art
In color processing (color management processing) utilizing a color profile, the general practice presently is to perform calibration as a correction for a change in the engine characteristics of a printer.
The flow of processing using an ordinary color profile will be described with reference to FIG. 1, by way of example. As shown in FIG. 1, input data 101 of CMYK values (C1, M1, Y1, K1) is converted to color-space data 103 in L*a*b* color space by performing a conversion using a source color profile 102. This L*a*b* color space is a device-independent color space with respect to input CMYK values. The color-space data 103 is converted to output data 105 of CMYK values (C2, M2, Y2, K2) by performing a conversion using a destination color profile 104. The destination color profile 104 contains combinations of CMYK values of an output device with respect to the color-space data 103.
The output data 105 is converted to data 106 of CMYK values (C2′, M2′, Y2′, K2′) by a calibration unit that corrects for a change in the tone characteristics of the engine. Here C2, M2, Y2 and K2 are converted to C2′, M2′, Y2′ and K2′, respectively, by one-dimensional LUTs (Look-Up Tables) 107, 108, 109 and 110, respectively.
Next, the flow of processing using a device-link profile that has recently come into circulation will be described with reference to FIG. 2. The device-link profile is a profile incorporating a correspondence table of correspondence from input values to output values created by finding the correspondence of output values from input values using color management on the basis of a profile for input and a profile for output. A color-space conversion from input values to output values can be performed using this profile alone.
The input data 101 shown in FIG. 2 is converted directly to output data 202 of CMYK values (C3, M3, Y3, K3) by a device-link profile 201. In a manner similar to that of FIG. 1, the output data 202 obtained by the conversion is converted to data 203 of CMYK values (C3′, M3′, Y3′, K3′) by a calibration unit that corrects for a change in the tone characteristics of the engine. That is, C3, M3, Y3 and K3 are converted to C3′, M3′, Y3′ and K3′, respectively, by one-dimensional LUTs 107, 108, 109 and 110, respectively.
The difference between the ordinary color profile and device-link profile will be described with reference to FIGS. 3 and 4. FIG. 3 is a diagram useful in describing color processing using the ordinary color profile, and FIG. 4 is a diagram useful in describing color processing using the device-link profile.
In the case of the ordinary color profile, the combination of CMYK at 301 in FIG. 3 differs from the combination of CMYK at 302. However, the color-space data obtained by the conversion using the source color profile 102 takes on the same values (L*=5, a*=b*=0) 303 in both cases. If black of the same color-space data 303 is converted by the destination color profile 104, output data 304 of CMYK values (C3, M3, Y3, K3) is obtained. That is, in a case where values 301 and 302 having different input CMYK values indicate the same Lab values, the output CMYK values will be the same. On the other hand, the device-link profile is capable of making the output data correspond to input data of different CMYK values.
As illustrated in FIG. 4, the device-link profile converts CMYK values of the input data directly to CMYK values of the output data. It is possible, therefore, for separate output CMYK values to be correlated with the input data 301 and input data 302 (i.e., for the input data 301 and the input data 302 to be handled separately). The chief feature is that if the input data differs, a conversion can be made in such a manner that the output data also will differ.
More specifically, with processing according to the ordinary color profile, the output with respect to black involves the same combination of CMYK. With the device-link profile, however, a difference in CMYK values in input data can be reflected directly in a difference in CMYK values in output data.
Recently, however, in order to raise the accuracy of calibration, C, M, Y, K are not corrected by respective one-dimensional LUTs. Rather, as illustrated in FIG. 5, an arrangement in which a correction is applied using a multi-dimensional LUT 503 has begun to be employed. With a one-dimensional LUT, what is reproduced solely with Y (yellow) before calibration is Y only even after calibration is performed; only Y is calibrated, and there is no change whatsoever in CMK.
As illustrated in FIG. 5, the multi-dimensional LUT 503 is capable of defining CMYK-to-CMYK. Therefore, output data can be converted to a combination of CMYK even with respect to Y-only input data. As a result, in this case the combination of colors cannot be assured, but the accuracy of color reproduction can be improved in comparison with the case where one-dimensional LUTs are used.
Nevertheless, a drawback is that even if a conversion is made to yellow-only data 601 using the device-link profile 201, as illustrated in FIG. 6, there is a possibility that as a result of the conversion by the multi-dimensional LUT 603, a color other than Y will mix in with data 602 after calibration.
It might not be hoped that a color mixes from accuracy of color reproduction as the type of an image and hope of a user.